Whenever I start up VS Code on my Mac (Yosemite), I get the following error in VS Code
Error: Cannot start Omnisharp because Mono version >=3.10.0 is required
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If you followed the instructions at the asp.net website, then you should already have brew installed. If you don't you can install it from this website: http://brew.sh
After you have that installed open a new Terminal and run the following:
brew update && brew install mono
Then you should be good to go. I just went through these same steps myself.